Some conventional filter circuits using an amplifier realize a filter characteristic using conductance of the amplifier and capacitance. According to this filter circuit, a positive-phase input terminal of a first conductance amplifier, which constitutes an active filter, is connected to a first input terminal via first resistor and connected to a ground point via second resistor. Furthermore, a negative-phase input terminal of the first conductance amplifier is connected to a second input terminal via third resistor and connected to an external output terminal via fourth resistor. Then, the resistance value ratio between the first resistor and second resistor is equalized to the resistance value ratio between the third resistor and fourth resistor and one signal including an in-phase signal component is added to the first input terminal and the other signal is added to the second input terminal. In this way, the conventional filter circuit adopts a multi-stage structure with each stage consisting of a capacitance and conductance amplifier, and can thereby obtain a steep attenuation characteristic.
However, the conventional filter circuit and radio apparatus realize nothing more than a filter in a low pass type configuration, and realizing a band pass type configuration in a high-frequency band requires a combination of a low pass type configuration and high pass type configuration, and furthermore, obtaining a steep attenuation characteristic requires a multi-stage configuration, and there are problems that the number of capacitances and conductance amplifiers increase and the size of the circuit increases, making integration of the circuit difficult. On the other hand, reducing the number of stages of the filter circuit to reduce the amount of attenuation of a desired frequency component without increasing the size of the circuit results in a problem that it is difficult to realize a steep attenuation characteristic or passage characteristic in a high frequency band.